Carbon nanotube arrays

ABSTRACT

A carbon nanotube array includes a plurality of carbon nanotubes aligned in a uniform direction. Each carbon nanotube has at least one line mark formed thereon.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent application,entitled “CARBON NANOTUBE ARRAYS AND MANUFACTURING METHODS THEREOF” withapplication Ser. No. 11/404,523, filed on Apr. 14, 2006 and related tocommonly-assigned applications, entitled “DEVICES FOR MANUFACTURINGCARBON NANOTUBE ARRAYS” with U.S. application Ser. No. 11/404,522, filedon Apr. 14, 2006 (Atty. Docket No. US7273) and “METHODS FOR MEASURINGGROWTH RATES OF CARBON NANOTUBES” with U.S. application Ser. No.11/404,700, filed on Apr. 14, 2006 (Atty. Docket No. US7274), thecontent of the above-referenced applications is hereby incorporated byreference.

BACKGROUND

1. Field of the Invention

The invention relates generally to carbon nanotubes and manufacturingmethods thereof and, more particularly, to a carbon nanotube array and amanufacturing method thereof.

2. Discussion of Related Art

Carbon nanotubes (also herein referred to as CNTs) are very smalltube-shaped structures essentially having a composition of a graphitesheet in a tubular form. Carbon nanotubes have interesting andpotentially useful electrical and mechanical properties and offerpotential for various uses in electronic devices. Carbon nanotubes alsofeature extremely high electrical conductivity, very small diameters(much less than 100 nanometers), large aspect ratios (i.e.length/diameter ratios) (greater than 1000), and a tip-surface area nearthe theoretical limit (the smaller the tip-surface area, the moreconcentrated the electric field, and the greater the field enhancementfactor). These features make carbon nanotubes ideal candidates forelectron field emitters, white light sources, lithium secondarybatteries, hydrogen storage cells, transistors, and cathode ray tubes(CRTs).

Generally, there are three methods for manufacturing carbon nanotubes.The first method is the arc discharge method, which was first discoveredand reported in an article by Sumio Iijima, entitled “HelicalMicrotubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp.56-58). The second method is the laser ablation method, which wasreported in an article by T. W. Ebbesen et al., entitled “Large-scaleSynthesis of Carbon Nanotubes” (Nature, Vol. 358, 1992, pp. 220). Thethird method is the chemical vapor deposition (CVD) method, which wasreported in an article by W. Z. Li, entitled “Large-scale Synthesis ofAligned Carbon Nanotubes” (Science, Vol. 274, 1996, pp. 1701).

In order to use the carbon nanotubes more widely and more effectively,it is necessary to implement a controlled growth of the carbon nanotubeswith desired structural parameters. Thus, it is pressing to unravel anunderlying growth mechanism of the carbon nanotubes. On the road towardunraveling the growth mechanisms of the carbon nanotubes, it is of vitalimportance to study the growth kinetics of the carbon nanotubes. Theprogress in the synthesis of a CNT array gave a convenience of studyingthe growth kinetics of the carbon nanotubes by measuring a height of theCNT array. This is because the CNT array is a self-ordered structure,and the carbon nanotubes of the CNT array are nearly parallel-aligned.

The above-mentioned arc discharge method and laser ablation method can'tsynthesize CNT arrays, while the above-mentioned CVD method cansynthesize CNT arrays. However, when adopting the conventional CVDmethod to synthesize a CNT array, the difficulty is that people do notreadily know when the carbon nanotubes start their growth and when thegrowth has ceased. Furthermore, people can't easily distinguish whetherthe carbon nanotubes are formed in the tip-growth mode or theroot-growth mode. Still furthermore, the growth rates of the carbonnanotubes at different temperatures can't be readily measured.

What is needed, therefore, is a carbon nanotube array from which peoplecan know when the carbon nanotubes start their growth and when thegrowth has ceased, be able to readily distinguish the growth mode of thecarbon nanotubes thereof, and can readily measure the growth rates ofthe carbon nanotubes at different temperatures.

What is also needed is a method for manufacturing the above-describedcarbon nanotube array.

SUMMARY

A carbon nanotube array includes a plurality of carbon nanotubes alignedin a uniform direction. Each carbon nanotube has at least one line markformed thereon.

Other advantages and novel features of the present carbon nanotube arrayand the related manufacturing method will become more apparent from thefollowing detailed description of preferred embodiments when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present carbon nanotube array and the relatedmanufacturing method can be better understood with reference to thefollowing drawings. The components in the drawings are not necessarilyto scale, the emphasis instead being placed upon clearly illustratingthe principles of the present carbon nanotube array and the relatedmanufacturing method.

FIG. 1 is a schematic, side view of a carbon nanotube array inaccordance with an exemplary embodiment of the present device, eachcarbon nanotube of the carbon nanotube array having a plurality of linemarks formed thereon;

FIG. 2 is a schematic, side view of an exemplary device adopted formanufacturing the carbon nanotube array of FIG. 1;

FIG. 3 includes a plot of gas flux versus time (FIG. 3A) in accordancewith a first exemplary embodiment of the present method and acorresponding Scanning Electron Microscope (SEM) image (FIG. 3B) of thecarbon nanotube array formed thereof; and

FIG. 4 includes a plot of gas flux versus time (FIG. 4A) in accordancewith a second exemplary embodiment of the present method and acorresponding Scanning Electron Microscope (SEM) image (FIG. 4B) of thecarbon nanotube array formed thereof.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate at least one embodiment of the present carbon nanotube arrayand the related manufacturing method, in one form, and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made to the drawings to describe embodiments ofthe present carbon nanotube array and the related manufacturing methodthereof.

FIG. 1 is a schematic, side view of a carbon nanotube array 10, inaccordance with an exemplary embodiment of the present device. As shownin FIG. 1, the carbon nanotube array 10 includes a substrate 12 and aplurality of carbon nanotubes 14 formed on the substrate 12. The carbonnanotubes 14 are aligned in a substantially uniform direction. In thepreferred embodiment, the carbon nanotubes 14 are substantiallyperpendicular to the substrate 12. Each carbon nanotube 12 has aplurality of line marks 16 separately formed thereon, and the carbonnanotube 12 is thereby divided into several, readily recognizablesegments.

FIG. 2 is a schematic, side view of an exemplary device 20 adopted formanufacturing the carbon nanotube array of FIG. 1. As shown in FIG. 2,the device 20 is a reaction furnace and includes a reaction chamber 220,a gas introducing tube 228, and a supporting component (i.e., quartzboat) 240. The reaction chamber 220 is a tubular container and is madeof high-temperature resistant material having steady chemical properties(e.g., quartz, alumina, or another high-temperature ceramic). Thereaction chamber 220 includes a first gas inlet 222, a second gas inlet224, and a gas outlet 226. The first gas inlet 222 and the second inlet224 extend through one end of the reaction chamber 220, and the gasoutlet 226 extends through the other opposite end of the reactionchamber 220. The first gas inlet 222 is used to introduce a reaction gasand the second gas inlet 224 is used to introduce a disturbance gas.

The quartz boat 240 is disposed in the reaction chamber 220. The quartzboat 240 is used to carry the substrate 12 thereon. As shown in FIG. 2,the quartz boat 240 is semi-closed, and has one opening (not labeled)and one closed end (not labeled) opposite to the opening. The openingfaces the first gas inlet 222. Alternatively, the quartz boat 240 can becymbiform (i.e., boat-shaped), tabulate/planar, and so on. The gasintroducing tube 228 is generally made of high-temperature resistantmaterial having steady chemical properties (e.g., quartz, alumina, oranother high-temperature ceramic). The gas introducing tube 228 includesa first end connected to the second gas inlet 224 and a second end,advantageously, connected to the quartz boat 240. The second end of thegas introducing tube 228 runs through the closed end of the quartz boat240 or at least extends to an opening therein and is aimed at thesubstrate 12. A distance between the substrate 12 and the second end ofthe gas introducing tube 228 is as small as possible. The gasintroducing tube 228 is used to transport the disturbance gas,introduced from the second gas inlet 224, to the quart boat 240.Alternatively, the second end of the second gas inlet 224 need notnecessarily intersect and/or be directly connected to the quartz boat240. What is required, however, is that the second end of the second gasinlet 224 is positioned sufficiently proximate to the substrate 12,carried by the quartz boat 240, to effectively deliver the disturbancegas thereto.

An exemplary method for manufacturing the carbon nanotube array 10 ofFIG. 1 includes the following steps: (a) providing a substrate 12; (b)forming a catalyst layer on the substrate 12; (c) heating the substrate12 to a predetermined temperature; and (d) intermittently introducing areaction gas to grow a patterned carbon nanotube array 10 upon thecatalyst layer.

In step (a), the substrate 12 is made of high-temperature resistantmaterial having steady chemical properties, such as glass, quartz,silicon, or alumina.

Furthermore, a protection layer (e.g., silicon dioxide) can be formed onthe substrate 12. A thickness of the protection layer is approximatelyin the range from 100 nanometers to 1000 nanometers. In the preferredembodiment, the substrate 12 is made of silicon. A protection layer madeof silicon dioxide is formed on the silicon substrate 12. A thickness ofthe silicon dioxide is approximately in the range from 400 nanometers to800 nanometers.

In step (b), the catalyst layer is uniformly disposed on the substrate110 by means of, e.g., chemical vapor deposition, thermal deposition,electron-beam deposition, or sputtering. A thickness of the catalystlayer is in the approximate range from 1 nanometer to 10 nanometers. Thecatalyst layer is a whole. Alternatively, the catalyst layer is dividedinto a plurality of portions separated from each other by, for example,a UV lithography-sputtering-lift off process. This is beneficial to formthe carbon nantobue array 10 with a uniform height. The catalyst layercan be made of iron (Fe), cobalt (Co), nickel (Ni), alloys thereof, oroxides including Fe, Co, and/or Ni. In the preferred embodiment, thecatalyst layer is made of iron (Fe). A thickness of the iron layer is inthe approximate range from 3 nanometers to 5 nanometers. In oneembodiment, the catalyst layer is annealed at an approximate temperaturein the range from 200° C. to 400° C. This is beneficial to obtain ironparticles. The iron particles tend to enhance the catalysis.

Then, the substrate 12, with the iron particles disposed thereon, isplaced in the reaction furnace 20. Furthermore, as shown in FIG. 2, asilica pad 230 is disposed in the quartz boat 240, and the substrate 12with the iron particles disposed thereon is placed on the silica pad230. The silica pad 230 is configured for facilitating the convenientremoval of the substrate 12 out of the quartz boat 240 upon completionof processing.

That is, the substrate 12 can be conveniently taken out of the quartzboat 240 by taking the silica pad 230 out of the quartz boat 240.Furthermore, the silica pad 230 is relatively flat and keeps thesubstrate 12 from engaging with the quartz boat 240 directly. Thisarrangement ensures that the substrate 12 is heated uniformly. Thisuniformity in heating is further beneficial to form the carbon nanotubearray 10 with a uniform height.

In step (c), a temperature in the reaction furnace is approximately inthe range from 500° C. to 900° C. In step (d), the intermittentintroduction of the reaction gas is executed by repeating a process ofproviding the introduction of the reaction gas for a first predeterminedtime and interrupting the introduction of the reaction gas for a secondpredetermined time. The first predetermined time is approximately in therange from 1 minute to 5 minutes. The second predetermined time isapproximately in the range from 10 seconds to 30 seconds.

Firstly, the first gas inlet 222 is open, and the reaction gas isintroduced from the first gas inlet 222 and into the reaction chamber220 for the first predetermined time. The reaction gas gets into thequartz boat 240 through the opening thereof to grow the array of carbonnanotubes 14 upon the substrate 12 via the catalyst layer. The reactiongas is a mixture of a carbon source gas and a protection gas. Theprotection gas is used to adjust a concentration of the carbon sourcegas. The carbon source gas can, for example, be ethylene (C₂H₄), methane(CH₄), acetylene (C₂H₂), or ethane (C₂H₆). The protection gas can be,e.g., argon (Ar) gas, nitrogen (N₂) gas, or hydrogen (H₂) gas. In thepreferred embodiment, the carbon source gas is acetylene (C₂H₂) and theprotection gas is argon (Ar) gas.

Then, the first gas inlet 222 is closed and the introduction of thereaction gas is switched off for the second predetermined time, therebyinterrupting the flow of the reaction (i.e., carbon source) gasproximate the substrate. This would result a distinct line mark 16 oneach as-grown carbon nanotube 14. In one embodiment, when theintroduction of the reaction gas is switched off, a disturbance gas canbe simultaneously introduced from the second gas inlet 224. Thedisturbance gas is transported into the quartz boat 240 through the gasintroducing tube 228. The disturbance gas can be reductive gases and/orinert gases, such as argon (Ar) gas, nitrogen (N₂) gas, and hydrogen(H₂) gas. The disturbance gas can rapidly blow off the residual carbonsource gas near the substrate 12, thus quickly halting the nanotubegrowth process. The described process of interrupting the introductionof the reaction gas (each such on/off cycle of the reaction gas, therebycorresponding to a growth cycle) is repeated a chosen number of times,and the carbon nanotube array 10 as shown in FIG. 1 is formed. Eachcarbon nanotube 14 of the carbon nanotube array 10 has a plurality ofline marks 16 formed thereon, with the number of distinct sectionswithin a given nanotube 14 corresponding to the number of growth cycles.

Alternatively, step (d) can be executed as follows. Firstly, the firstgas inlet 222 is open, and the reaction gas is introduced from the firstgas inlet 222 and into the reaction chamber 220 for the firstpredetermined time. The reaction gas gets into the quartz boat 240through the opening thereof to grow the array of carbon nanotubes 14from the substrate 12.

Then, the second gas inlet 224 is open, and a disturbance gas isintroduced from the second gas inlet 224 for the second predeterminedtime. The disturbance gas can rapidly blow off the carbon source gasnear the substrate 12, thereby interrupting the flow of the carbonsource gas proximate the substrate. This would result a distinct mark 16on each as-grown carbon nanotube 14. The described process ofinterrupting the introduction of the reaction gas (each such on/offcycle of the disturbance gas thereby corresponding to a growth cycle) isrepeated a chosen number of times, and the carbon nanotube array 10 asshown in FIG. 1 is formed. Each carbon nanotube 14 of the carbonnanotube array 10 has a plurality of line marks 16 formed thereon. Ascan be seen from FIG. 3B, the marks 16 clearly indicate that there is astructural variant to the individual carbon nanotubes 14 at the area ofthe stop/start growing points. This variant can be seen on all of thecarbon nanotubes 14 in the carbon nanotube array 10 by the line marks16.

A plot of gas flux versus time (FIG. 3A) in accordance with a firstexemplary embodiment of the present method and a corresponding ScanningElectron Microscope (SEM) image (FIG. 3B) of the carbon nanotube array10 formed thereof are shown. The growth temperature is about 933K, andthe growth time is about seventeen minutes. Firstly, the first gas inlet222 is open, and the reaction gas (i.e., a mixture of ethylene (C₂H₄)and argon (Ar) gas) is introduced from the first gas inlet 222 and intothe reaction chamber 220 for about 5 minutes. Then, the second gas inlet224 is open, and a disturbance gas (i.e., argon (Ar) gas) is introducedfrom the second gas inlet 224 and into the reaction chamber 220 forabout 10 seconds. After that, the sequence is executed as follows:introducing the reaction gas for about 1 minute, introducing thedisturbance gas for about 10 seconds, introducing the reaction gas forabout 2 minutes, introducing the disturbance gas for about 10 seconds,introducing the reaction gas for about 3 minutes, introducing thedisturbance gas for about 10 seconds, introducing the reaction gas forabout 1 minute, introducing the disturbance gas for about 10 seconds,introducing the reaction gas for about 2 minutes, introducing thedisturbance gas for about 10 seconds, introducing the reaction gas forabout 3 minutes, and introducing the disturbance gas for about 10seconds.

As shown in FIG. 3B, the resulting carbon nanotube array is divided intosegments with different lengths by a series of parallel line marks. Bymatching theses segments with the mark sequence shown in FIG. 3A, it'sunambiguously shown that growth points of the carbon nanotubes are atthe bottom of the carbon nanotube array.

That is, the carbon nanotubes are synthesized in the root-growth mode.Furthermore, the tops of the carbon nanotubes are the first grown parts.Further as shown in FIG. 3B, the last intended line mark does notappear. Thus, it can be concluded that the carbon nanotubes terminatetheir growth within the second 3-minute-growth interval.

The specific time when the carbon nanotubes start and cease growth areunknown. Thus, when measuring the growth rate of the carbon nanotubes,the first and last segments of the carbon nanotube array are neglected.The growth rate is equal to the length between a pair of line marksdivided by the time interval between such two line marks.

Alternatively, a plot of gas flux versus time (FIG. 4A) in accordancewith a second exemplary embodiment of the present method and acorresponding Scanning Electron Microscope (SEM) image (FIG. 4B) of thecarbon nanotube array 10 formed thereof are shown. The growthtemperature is about 933K, and the growth time is about one hour. Adisturbance gas (i.e., argon (Ar) gas) is introduced from the second gasinlet 224 and into the quartz boat 240 and near the substrate 12 aboutevery 5 minutes. The total number of line marks is eleven. As shown inFIG. 4A, the as-grown carbon nanotube array should have twelve segmentsseparated by the eleven line marks. However, as shown in FIG. 4B, theresulting carbon nanotube array actually has eight segments separated byseven line marks. That is, the carbon nanotubes terminate their growthwithin after the seventh and before the eighth mark time (i.e., thenanotubes stop growing within the eighth growth cycle).

The eight growth time cycles are substantially equal, and the lengths ofthe eight segments are substantially equal. Because the specified growthtime in the first and last 5 minutes are uncertain, the first and lastsegments of the carbon nanotube array are neglected when measuring thegrowth rate of the carbon nanotubes. The growth rate is equal to thelength between a pair of line marks divided by 5 minutes.

Compared with the conventional CVD method, the present method can beused to manufacture the CNT array with a plurality of line marks andthus distinct sections. From this CNT array, people can readilydistinguish the carbon nanotubes are the root-growth mode. Furthermore,people can easily know when the carbon nanotubes start their growth andwhen the growth has ceased. Still furthermore, the growth rates of thecarbon nanotubes at different temperatures can be readily measured.

Finally, it is to be understood that the above-described embodiments areintended to illustrate rather than limit the invention. Variations maybe made to the embodiments without departing from the spirit of theinvention as claimed. The above-described embodiments illustrate thescope of the invention but do not restrict the scope of the invention.

1. A carbon nanotube comprising at least one mark formed by variance inthe structural configuration of the carbon nanotube.
 2. The carbonnanotube as claimed in claim 1, wherein the carbon nanotube has aplurality of marks.
 3. The carbon nanotube as claimed in claim 1,wherein the carbon nanotube has a number of distinct sections divided bythe at least one mark formed thereon.
 4. The carbon nanotube as claimedin claim 3, wherein the distinct sections have different lengths.
 5. Thecarbon nanotube as claimed in claim 3, wherein the number of distinctsections corresponds to a number of growth cycles of the carbonnanotube.
 6. The carbon nanotube as claimed in claim 5, wherein each ofthe growth cycles of the carbon nanotube corresponds to an on and offcycle of a reaction gas for growing the carbon nanotube.
 7. The carbonnanotube as claimed in claim 5, wherein each of the growth cycles of thecarbon nanotube corresponds to an on and off cycle of a disturbance gasintroduced during a growing step of the carbon nanotube.
 8. A carbonnanotube comprising of at least one structural variance caused by adisruption of the carbon nanotube's growth.
 9. The carbon nanotube asclaimed in claim 8, wherein the carbon nanotube has a number of distinctsections divided by the at least one structural variance formed thereon.10. The carbon nanotube as claimed in claim 9, wherein the distinctsections have different lengths.
 11. The carbon nanotube as claimed inclaim 9, wherein the number of distinct sections corresponds to a numberof growth cycles of the carbon nanotube.
 12. The carbon nanotube asclaimed in claim 11, wherein each of the growth cycles of the carbonnanotube corresponds to an on and off cycle of a reaction gas forgrowing the carbon nanotube.
 13. The carbon nanotube as claimed in claim11, wherein each of the growth cycles of the carbon nanotube correspondsto an on and off cycle of a disturbance gas introduced during a growingstep of the carbon nanotube.
 14. A carbon nanotube comprising of atleast one structural variance in the alignment of the carbon.